A digital coherent reception technique has been interested as a technique of implementing large-capacity optical communication with long-distance transmission. In the digital coherent reception technique, intensity information and phase information of a received signal are extracted using a coherent front-end circuit according to a coherent optical reception scheme using a local oscillation light. Then, compensation for waveform distortion occurred in a transmission path is performed on the received signal by using digital signal processing based on the extracted intensity information and the extracted phase information, and then the received signal is demodulated.
In the case where coherent optical reception is employed, an apparatus that cancels a frequency error (frequency offset) between the received signal and the local oscillation light is important.
(Frequency Offset Compensation)
As a method of cancelling a frequency offset between a received signal and local oscillation light, a technique proposed in, for example, JP 2009-135930 A has been known. This technique performs: detecting a frequency offset from a received digital signal; and compensating the frequency offset by applying an opposite phase rotation corresponding to the detected frequency offset to the received signal.
(Frequency Offset Estimation)
Meanwhile, as a frequency offset estimation method, techniques proposed in Andreas Leven et al., Frequency Estimation in Intradyne Reception, IEEE Photonics Technology Letters, Vol. 19, No. 6, Mar. 15, 2007, pp. 366-368 and JP 2009-130935 A have been known.
(N-th Power Method)
In the estimation technique disclosed in Andreas Leven et al., a phase rotation amount per delay amount is calculated while cancelling a phase noise by performing delay, conjugation and multiplication on a baseband electrical signal (complex signal) of an input N phase PSK signal, and a phase rotation amount per delay amount due to a frequency offset is obtained by raising an obtained signal to the power of N and cancelling a data phase (modulated phase component). Here, N represents a multi-valued degree, and N is 4 in the QPSK. In the QPSK, a possible value of a data phase is any one of 0, ±π/2, ±π, and ±3π/4.
Here, the data phase is cancelled by the above N-th (=4)-power, however, the frequency offset quadruples. After influence of noise is cancelled by averaging, the frequency offset is subjected to a ¼ argument operation by a ¼ argument calculation, and so a frequency offset estimation value is obtained.
(PADE Technique)
Meanwhile, in the estimation technique discussed in JP 2009-130935 A, unlike the N-th power method described above, a conjugate calculation and an N-th power of a complex signal are not used, and an estimatable range of a frequency offset is increased to be larger than in the N-th power method. This estimation technique is also called a pre-decision based angle differential frequency offset estimator (PADE).
In the PADE technique, in order to remove a symbol phase term (nπ/4 (n=1, 2, 3, 4) in the case of the QPSK), a provisional determination of a symbol phase value is performed by using a laser phase noise estimation amount and a frequency offset estimation value preceded by a one-symbol time to remove the symbol phase term.
The N-th power method is a technique supporting only the PSK. Meanwhile, the PADE technique can support all modulation schemes by using a provisional determiner as an identification circuit supporting the respective modulation schemes.
However, when the frequency offset estimation value is significantly different from an actual frequency offset, it is difficult to set an appropriate frequency error to the provisional determiner, and an error occurs in the provisional determination. For example, as illustrated in FIG. 13, when there is a frequency estimation error, in a determination threshold value of the 16QAM, it is erroneously determined as an adjacent symbol, and thus a phase error is smaller than a true phase error, or it looks like there is no error.
As a result, in the phase and amplitude shift keying scheme such as the 16QAM, the frequency offset estimation value may be unstable, and thus the value may converge with an erroneous frequency offset estimation value, or the tracking performance to variation in a frequency offset significantly may deteriorate.